Flexibility and plasticity in neuronal signalling is a property of the central nervous system which allows the individual to adapt to its surroundings and to change its behavior according to the past experience. An example of such a plastic phenomenon is postactivation potentiation which is characterized by an increased neuronal activity to a single stimulus following brief, intense synaptic activation. It is an attractive model of prolonged modification of synaptic efficacy which could be induced not only by electrical stimulation but also by a training procedure. The dentate fascia (of the hippocampal formation) yields such post-activation potentiation lasting for hours, even days, following stimulation of its afferent pathway (the perforant path). In such an experimental arrangement, we have shown long-lasting changes in the fine structure of the dentate fascia (enlargement of dendritic spines in the dentate molecular layer by 38%) which was observed 2 minutes after stimulation and outlasted the stimulus by 24 hours. Therefore, it has been postulated that spine enlargement is the mechanism underlying postactivation potentiation. If this were so, then given the increased activity in the dentate during task acquisition, a morphological change similar to that of postactivation potentiation could be also expected as a result of a training experience. Indeed, such a change, although similar in magnitude (spines were enlarged by 20%), has been shown in preliminary behavioral experiments using an appetitive conditioning paradigm. It is proposed to bring further evidence on the causal relation between these two phenomena by demonstrating that spine enlargement is specifically related to the training procedure and not to the stress which might accompany this procedure. An attempt will also be made to verify the hypothesis that increased metabolic activity and protein synthesis underly the mechanism of long-lasting spine enlargement, be it elicited by postactivation potentiation or by conditioning. This proposal is ultimately aimed at the demonstration of morphological changes underlying certain phases of process of learning and memory.